A. Field of Invention
This invention pertains to the art of methods and apparatuses for providing components for use in hostile thermal environments. More particularly, the invention is directed to high heat flux devices having two or more structural layers.
B. Description of the Related Art
Current combustion chambers and related devices used in hydrogen-oxygen fueled rockets, or advanced air breathing “combined cycle” designs are placed in very high heat flux environments. As such, they are usually made out of copper alloys to take advantage of copper's high thermal conductivity, or from nickel-base superalloys, to take advantage of their high temperature strength. For a given component, choice of the preferred material (copper or superalloy) depends on the actual heat flux environment and the trade-off between strength and conductivity. Copper alloys have very high conductivity (300 W/mK) but only moderate strength and are limited by oxidation attack to temperatures of about 1200° F. Superalloys have the strength and oxidation resistance to reach as high as 2200° F., but have low conductivity, (10 W/mK). NiAl has oxidation resistance up to 2400° F. and is stronger than many superalloys, especially wrought alloys, above 1900° F. It has thermal conductivity value of about 70 W/mK, lower than copper but seven times better than superalloys.
NiAl is an intermetallic compound that has been studied for potential turbine engine applications for many years. One extensively used form of NiAl is as a thin coating (less than 5% of the total part thickness) for superalloy turbine blades. As a coating, NiAl is not a structural (load-bearing) component, but is applied to provide resistance to oxidation and corrosion attack.
Additionally, NiAl has been examined as a structural material. It is known in the art to employ alloys as structural parts in jet engines. The benefits of NiAl in general and these alloys specifically are their lighter weight, higher melting point, good environmental resistance, and high thermal conductivity (relative to superalloys, not copper). However, NiAl alloys are not currently used in structural applications in actual engines because they are excessively brittle and are not damage tolerant. U.S. Pat. No. 5,516,380 discusses the low ductility of binary NiAl intermetallics that impedes the implementation of NiAl intermetallics as a viable substitute for nickel-base superalloys.
U.S. Pat. No. 5,725,691 discloses alloys for use in structural applications based on NiAl. Selected elements are added to the alloy to enhance room temperature ductility and high temperature strength.
Bimetallic structures have also been widely identified for engineering applications. The bimetallic concept is different from a coating in that the thickness of the two metallic components may each approach 50% of the total part thickness, rather than one component being confined to a thin coating layer. For example, many electronic sensors and switches are bimetallic components. Bimetallic concepts in jet engines have been considered in the past, but the time/temperature combinations typical of commercial jet aircraft engines are such that the bimetallic structures may degrade due to interdiffusion.
U.S. Pat. No. 5,975,852 discloses a thermal barrier coating and a method for forming the coating on an article designed for use in a hostile thermal environment. This patent discusses the use of a NiAl substrate to which a thermal barrier coating is applied. Alternately, the patent discloses that the substrate may be formed of a superalloy on which the NiAl alloy is provided as a monolithic surface layer having a thickness of at least 125 micrometers. However, each of the disclosed applications includes a ceramic layer covering a layer of aluminum oxide.
U.S. Pat. No. 5,161,898 discloses a roller cutter drill bit having bearing elements formed of a superalloy material. The bearing surfaces of the bearing elements have an aluminide coating to purportedly accommodate applications with temperatures up to and above 500° F. and thermal load spikes of 1000° F. The disclosed aluminide coating comprises a thickness of preferably around 0.005 inch on a component having a total thickness of 0.050 inch.
Japanese Publication 09-157866 discloses a corrosion resistant and oxidation resistant coating film for application onto a substrate base material. The disclosed film includes a single phase NiAl layer applied either directly to the substrate or onto an intermediate CoNiCrAlY layer. The disclosed coatings ranged in thickness from 0.10 mm (0.004 inch) to 0.30 mm (0.012 inch) (Table 1).
U.S. Pat. No. 3,625,750 discloses a method for producing aluminum intermetallic coatings on articles consisting of nickel- or cobalt-based alloys. An exemplary coating ranges from 1.75 mils (0.00175 inch) to 3.5 mils (0.0035 inch) on the surface of a turbine blade.
U.S. Pat. No. 5,348,446 discloses an airfoil for a gas turbine engine. The conventional airfoil profile is constructed from a core body formed of a conventional nickel-based superalloy. Leading and trailing edge components and a squealer tip formed of a nickel aluminide alloy are bonded to the core body.
Japanese Publication 19661P-0067422 discloses a molybdenum or molybdenum alloy substrate having a multi-layered coating of chrome and nickel or nickel alloy and alumina. However, the molybdenum or molybdenum alloy is not innately resistant to oxidation at elevated temperatures. The coating serves as a necessary barrier to oxidation and serves no structural function.
U.S. Pat. No. 5,965,274 discloses an electronic circuit component comprising a NiAl and/or Ni3Al substrate upon which an alumina layer is formed prior to applying the conductive elements. The thermal conductivity of the electronic component may be enhanced by roll-bonding or joining a metal such as copper or copper-aluminum alloy to the substrate. The reference does not provide that the copper or copper-aluminum alloy forms a structural layer of the electronic component. Further, the reference does not provide for applications that involve exposure to hostile thermal environments.
U.S. Pat. No. 5,626,462 discloses an airfoil for use at high temperatures. The airfoil has a double-wall construction. The airfoil includes an airfoil support wall and an airfoil skin. A series of internal channels are formed within the double-wall airfoil structure between the airfoil support wall and the airfoil skin.
There remains a need in the art to provide components for use in high temperature applications having improved physical properties and environmental resistance. None of the available art provides a bimetallic or multi-layered structural component for use in high heat flux and hostile thermal environments where each layer is structural, i.e. load-bearing, comprising at least about 20% of the structural thickness. The present invention is directed to utilization of NiAl or NiAl-based alloy as a structural layer in a bimetallic or multi-layered component for use in hostile thermal environments in order to provide advantages over structural components known in the prior art.